U.S. Republican presidential candidate former U.S. House Speaker Newt Gingrich speaks to employees of the Al-Jon Company in Ottumwa, Iowa, December 20, 2011. REUTERS/Jeff Haynes

“I think that Congressman Paul has to explain his own situation and how he could have had a decade of newsletters that had his name on it that he apparently wasn’t aware of,” Gingrich said.

“I think that somebody should say to him ‘OK, how much money did you make from the newsletters?’ These things are really nasty, and he didn’t know about it? Wasn’t aware of it? But he’s sufficiently ready to be president? It strikes me it raises some fundamental questions about him.”

Paul, leading the race for the January 3 Republican caucuses vote in Iowa, the first nominating contest in the nation, has come under pressure after revelations of possible links to far-right comments.

Protesters disrupt a news conference by U.S. Republican presidential candidate and former U.S. House Speaker Newt Gingrich after he got the endorsement of Iowa Speaker of the House Kraig Paulsen and New Hampshire Speaker of the House Bill O'Brien at the Iowa State Capitol in Des Moines, Iowa, December 21, 2011. REUTERS/Jeff Haynes

A direct-mail solicitation for Paul’s political and investment newsletters in the 1990s warned of a “coming race war in our big cities” and of a “federal-homosexual cover-up” to play down the impact of AIDS.

The eight-page letter, which appears to carry Paul’s signature at the end, also warns that the U.S. government’s redesign of currency to include different colors - a move aimed at thwarting counterfeiters - actually was part of a plot to allow the government to track Americans using the “new money.”

Paul’s campaign has launched a wave of attack ads on Gingrich in Iowa, as the Republican race to select a nominee to challenge President Barack Obama in the 2012 election heats up.

PAYROLL TAX DEAL

Speaking before a crowd of about 250 in the early voting state South Carolina, Gingrich criticized Congress’s last-minute deal this week to extend the payroll tax extension for two months, which followed a bruising political battle.

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“I don’t know how we get this message across to both parties, but there’s something profoundly wrong in this economy, with the problems around the world threatening to make it worse, to have the president and Congress thinking that they accomplished something by passing a two-month extension of the payroll tax cut.”

He went on to tout his own record as speaker of the House of Representatives in the mid-1990s when, he said, he was able to work with Democrats on welfare reform, a balanced budget and the creation of 11 million new jobs. “Unemployment went down to 4.2 percent” during his tenure as speaker, he said.

Gingrich is the choice for 38 percent of South Carolina primary voters, while twenty-one percent said they favored former Massachusetts governor Mitt Romney, according to Clemson University poll results released on Monday.

The poll surveyed by telephone 600 South Carolinians who said they would vote in the state’s January 21 primary. About a third of the respondents said they had decided on a candidate. The poll’s margin of error is plus or minus 4.5 percentage points.

Gingrich called Romney a “Massachusetts moderate”.

Fresh from his effort to get his name on the ballot for the Virginia primary, Gingrich will be off the campaign trail until Tuesday.